CO Adlayer Research Articles

Dipole–dipole coupling effects upon infrared spectroscopy of compressed electrochemical adlayers: Application to the Pt(111)/CO system

Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real-space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band-intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole-coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well-known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can account semiquantitatively for the marked higher intensity of the atop relative to the threefold hollow C–O stretching band in the observed infrared spectra even though the occupancy on the latter site is twofold greater and the singleton frequencies are substantially (∼280 cm−1) different. This coupling-induced intensity transfer toward the higher-frequency band component is likely to be a widespread phenomenon for densely packed adlayers. For the (2×2) adlayer, however, the isotope composition-dependent spectral band frequencies and relative intensities deviate markedly from the experiment. While the inclusion of stochastic broadening effects associated with adlayer disorder improves the situation, a satisfactory fit between theory and experiment requires the incorporation of vibrational coupling associated with short-range intermolecular interactions. For the (√19×√19) adlayer, on the other hand, dipole–dipole coupling with stochastic broadening accounts well for the observed spectral behavior. The more pronounced limitation of the conventional theory for the (2×2) structure may well be due to the abnormally high adsorbate packing density enhancing the importance of short-range interactions.

Electrochemical and in situ FTIRS studies of the CO adsorption at palladium and rhodium multilayers deposited on platinum single crystal surfaces II. Pt(100) substrate

Multilayers of rhodium and palladium have been deposited on Pt(100) electrodes and subsequently characterized by means of cyclic voltammetry and charge displacement experiments. In addition, the saturated adlayer of CO on PdPt(100) and RhPt(100) has been studied by both electrochemical and infrared techniques. The voltammetric profile corresponding to these multilayers is due to a coupled adsorption/desorption process involving both anions and hydrogen. The set of different results seems to indicate that the multilayer is epitaxial in the case of palladium, thus behaving as a bulk Pd(100) electrode, whereas it is disordered in the case of rhodium. The spectra for the saturated CO adlayer on RhPt(100) and PdPt(100) also reflect the presence of the foreign metal deposit and give some indication about the bonding geometry and order of the CO adlayer. The CO saturation coverage for the PdPt(100) system is around 0.63.

Reactive oxygen atoms on Cu(110) formed at 100 K: vibrational spectra and CO oxidation

The dissociative adsorption of oxygen on Cu(110) at 100 K and its reactivity for CO oxidation were studied by HREELS, TPD and LEED. Oxygen dissociatively adsorbs on Cu(110) at 100 K. These oxygen adatoms, designated as the as-exposed oxygen, form no ordered structure. Annealing this surface above 200 K causes the appearance of the (2 × 1)-O ordered structure which corresponds to the added row type reconstruction. A characteristic change in the loss feature of ν(CuO) was found upon annealing Cu(110) covered with the as-exposed oxygen by HREELS, which corresponded to the change from the as-exposed oxygen to oxygen atoms in the (2 × 1)-O phase. The as-exposed oxygen on Cu(110) at 100 K oxidized CO to form CO 2, while oxygen atoms in the (2 × 1)-O phase were inactive in TPD conditions. It was found that the reactivity of the as-exposed oxygen was more than 25 times higher than that of oxygen in the (2 × 1)-O phase. Four peaks at 125 K (α), 150 K (β), 180 K (γ) and 210 K (δ) were found in the CO 2 TPD spectra. The features of these peaks were discussed based on TPD spectra with various CO coverages at constant coverages of oxygen. In the coadlayer of CO and oxygen, CO species with a ν(CuCO) peak at 285 cm −1 was observed in addition to the ν(CuCO) peak at 340 cm −1 observed for a pure CO adlayer on Cu(110). Both the as-exposed oxygen and the oxygen in the (2 × 1)-O phase caused this downward shift.

Local structure and phase transitions within ordered electrochemical adlayers: Some new insights from in situ scanning tunneling microscopy

The application of in situ atomic-resolution scanning tunneling microscopy (STM) to the elucidation of local structure and potential-induced phase transitions within ordered adlayers on monocrystalline metal electrodes is discussed, and illustrated primarily for compressed CO adlayers on Pt(111). The value of the additional structural information on adsorbate binding sites that can be obtained from infrared reflection-absorption spectroscopy (IRAS) as well as from potentiodynamic STM imaging tactics is emphasized. The atomic-level and nanoscale structural alterations wrought upon the Pt(111)/CO adlayer by bismuth predosing are described as a further illustration of the synergetic value of the spatial and surface-bonding information provided by STM and IRAS. A comparison is made between the potential-dependent adlayer characteristics of the compressed CO adlayers with iodide adlayers on gold and platinum low-index electrodes. Possible reasons for the marked differences in the phase-transition behavior of these nonmetallic adlayers are briefly discussed, along with comparisons with analogous adlayers in ultrahigh vacuum environments.

The determination of the decomposition mechanism and kinetics of methoxy on clean Ni(100) and the effects of coadsorbed hydrogen, CO, and sulfur by time-resolved Fourier transform infrared spectroscopy

Time-resolved Fourier transform infrared spectroscopy (TR-FTIR) was used to simultaneously examine the mechanism and kinetics of methoxy decomposition on Ni(100) under varying conditions. The use of TR-FTIR to obtain kinetic parameters was first validated by comparison to temperature-programmed desorption using annealed CO adlayers. Assuming a first-order reaction with a constant pre-exponential factor of 5.2 × 10 12 s −1, we find the activation energy for methoxy decomposition to CO and hydrogen to increase as the reaction proceeds according to 16.9 + 10 θ CO ± 0.3 kcal/mol, where θ CO is the coverage of CO product in monolayers, for initial methoxy coverages between 0.18 and 0.056 ML. The addition of 0.10 ± 0.02 ML of CO increases the energy for CH bond activation to 19.2 ± 0.4 kcal/mol. Coadsorbed sulfur was found to inhibit both methoxy formation and decomposition. The addition of 0.14 ± 0.02 ML of sulfur raises the activation energy for methoxy decomposition by 1 ± 0.4 kcal/mol, an effect similar in magnitude to coadsorbed CO. The preadsorption of 0.4 ML of hydrogen reduces the amount of methoxy formed from a methanol multilayer but does not raise the activation energy for methoxy decomposition. These results are compared to methoxy decomposition studies on Ni(111) and Ni(110).

Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: some implications for ionic and chemisorbate solvation at electrode surfaces

The utility of infrared reflection-absorption spectroscopy (IRAS) for examining structure and bonding for model electrochemical interfaces in ultrahigh vacuum (UHV) is illustrated, focusing specifically on the solvation of cations and chemisorbed carbon monoxide on Pt(111). These systems were chosen partly in view of the availability of IRAS data (albeit limited to chemisorbate vibrations) for the corresponding in-situ metal-solution interfaces, enabling direct spectral comparisons to be made with the “UHV electrochemical model” systems. Kelvin probe measurements of the metal-UHV surface potential changes (ΔΦ) attending alterations in the interfacial composition are also described: these provide the required link to the in-situ electrode potentials as well as yielding additional insight into surface solvation. Variations in the negative electronic charge density and, correspondingly, in the cation surface concentration (thereby mimicking charge-induced alterations in the electrode potential below the potential of zero charge) are achieved by potassium atom dosage onto Pt(111). Of the solvents selected for discussion here — deuterated water, methanol, and acetonitrile — the first two exhibit readily detectable vibrational bands which provide information on the ionic solvation structure. Progressively dosing these solvents onto Pt(111) in the presence of low potassium coverages yields marked alterations in the solvent vibrational bands which can be understood in terms of sequential cation solvation. Comparison between these spectra for methanol with analogous data for sequential methanol solvation of gas-phase alkali cations enables the influence of the interfacial environment to be assessed. The effects of solvating chemisorbed CO are illustrated for acetonitrile; the markedly larger shifts in CO frequencies and binding sites for dilute CO adlayers can be accounted for in terms of short-range coadsorbate interactions in addition to longer-range Stark effects. The differing degrees of selective solvation of cations versus CO for water and acetonitrile are also explored. While the former solvent removes all specific effects of K + upon CO, these short-range interactions partly remain upon acetonitrile solvation. The close connections between the UHV-based findings and the behavior of the in-situ electrochemical systems are also discussed.

STM study of the Ni(110)-(2 × 1)-2CO system: structure and bonding-site determination

The structure of the Ni(110)-(2 × 1)-2CO system has been studied with scanning tunneling microscopy (STM) at room temperature. Contrary to many previous studies, it is found that atomically-resolved STM imaging of CO molecules is attainable. By comparing the apparent CO adlayer registry with coadsorbed low concentrations of O and S, it is found that the CO molecules bond in short-bridge sites along the Ni [11̄0] rows and alternately tilt away from the normal direction within the (001) plane.

CO adsorption on stepped Pd (112): studies by thermal and electron stimulated desorption

The adsorption of CO on a stepped Pd(112) surface, Pd(S)[3(111) × (001)], at 300 K has been studied by temperature programmed desorption (TPD) , electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED) methods. CO adsorbs molecularly on Pd (112) at 300 K, reversibly desorbing near 465 K at low coverages with first-order desorption kinetics. The activation energy and pre-exponential factor of CO thermal desorption are observed to decrease strongly with increasing CO coverage, exhibiting a logarithmic compensation effect. Evidence is presented that CO preferentially adsorbs at the step sites of Pd (112) at low coverages, and that both terrace and step sites are occupied at higher coverages. The cross-section for production of positive ions during electron bombardment of adsorbed CO depends strongly on the CO coverage. No excited-state neutral particles were detected at any CO coverage during ESD measurements. In addition, no evidence for long-range ordering of the CO adlayers on Pd (112) was observed.

Carbon monoxide adlayer structures on platinum (111) electrodes: A synergy between in-situ scanning tunneling microscopy and infrared spectroscopy

The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in-situ scanning tunneling microscopy (STM) along with infrared reflection–absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well-studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real-space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding-site assignments. In turn, the latter data provide an important crosscheck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturated CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. 0 V vs SCE observed previously under these conditions by IRAS. At potentials below 0 V, a hexagonal close-packed (2×2)–3CO adlayer is observed, with a CO coverage, θCO, of 0.75. The z-corrugation pattern evident in the STM images indicates the presence of two threefold hollow and one atop CO per unit cell. This binding-site assignment is supported by the corresponding IRAS data which yield C–O vibrational bands at ca. 2065 and 1775 cm−1. The relative intensities of these two νCO bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects between the atop and multifold CO oscillators. At potentials above 0 V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly different adlayer arrangement is formed, having a (√19×√19)R23.4°–13CO unit cell, with θCO=13/19. This hexagonal structure features CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (√19×√19) symmetry was achieved on the basis of the z-corrugation pattern along with the corresponding IRAS data upon consideration of dipole-coupling effects. Another CO adlayer structure, having a (√7×√7)R19.1°-4CO unit cell (θCO=4/7), was commonly observed at potentials below 0.2 V after the removal of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The increased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface potentials (and excess electronic surface charges) characterizing the electrochemical interface.

Coverage dependence of CO dissociation on clean and hydrogen presaturated Fe(100) surface

A detailed thermal desorption spectroscopy (TDS) study of very low and intermediate exposures of CO on clean and H 2 presaturated Fe(100) surfaces was conducted. At extremely low CO exposures, CO adsorbs only in the π-bonded CO state at low temperature. Molecules in this state exhibit an extremely weak carbon-oxygen bond and exclusively undergo dissociation when the temperature is raised above 440 K. At higher coverages, both dissociation and desorption is observed around 440 K even though all the molecules are still bound in the π-bonded CO state at lower temperatures. A model describing the partitioning between desorption and dissociation is proposed in which the dissociation fragments displace the strongly bound CO in the π-bonded state. The stoichiometry of this reaction allows a maximum of one half of π-bonded CO molecules to dissociate. Preadsorbed hydrogen strongly influences the ratio of dissociation to desorption as a function of coverage and suggests that hydrogen may induce island formation in the CO adlayer.

Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: The archetypical case of carbon monoxide/water coadsorption on Pt(111)

Infrared reflection-absorption spectroscopic (IRAS) along with work-function measurements are presented for carbon monoxide and deuterated water co-dosed onto Pt(111) at 110 K in ultrahigh vacuum (uhv) in comparison with coverage- and potential-dependent infrared spectra for the analogous in situ Pt(111)-aqueous electrochemical interface. The twin objectives are to assess the manner and degree to which the former ‘‘uhv electrochemical modeling’’ tactic mimics the vibrational spectral properties of the latter interface, and to exploit the additional molecular structural information obtainable for the former surface configuration in elucidating solvation effects for the in situ electrochemical system. Postdosing water onto CO adlayers in uhv yields progressive attenuation of the C–O stretching (νCO) IRAS bands associated with atop and twofold bridging coordination on the clean Pt(111) surface, being replaced by bands at lower frequencies that are more characteristic of the CO coverage (θCO)-dependent νCO features observed for the electrochemical interface. This ‘‘titration-like’’ spectral response, indicating that the added water assembles into hydrogen-bound islands which are nonetheless microscopically intermixed with the contiguous CO adlayer regions, is typically complete by water coverages corresponding to about 2–3 ‘‘equivalent bilayers.’’ Water dosage-dependent spectra in the O–D stretching (νOD) region for zero or low predosed CO coverages display the hallmarks of markedly increased hydrogen bonding beyond the bilayer point, whereas such hydrogen bonding is prevalent even at submonolayer water dosages for intermediate or near-saturated θCO values. The work-function decreases, −ΔΦ, induced by water dosing also are indicative of synergistic water/CO interactions, larger −ΔΦ values being obtained in the presence of low or intermediate predosed CO coverages than in its absence. A distinction is drawn between ‘‘specific’’ and ‘‘nonspecific’’ water co-dosing effects upon the CO adlayer structure; the former involves adjacent CO/D2O coadsorbate interactions, while the latter is describable most simply by the changes in the surface potential. The θCO-dependent CO site occupancies and νCO frequencies for the hydrated uhv and electrochemical Pt(111) systems are found to be in close concordance, the latter when the interfaces are compared at equivalent values of the surface potential. The findings thereby support a close structural connection between analogous low-temperature hydrated uhv and ambient-temperature electrochemical interfaces.

Anisotropy and coverage dependence of CO surface diffusion on Ru(S)-[15(001) × 2(100)

The surface diffusion of CO on Ru(S)-[15(001) × 2(100)] was studied using laser-induced thermal desorption (LITD) techniques. CO diffusion on the stepped Ru(001) surface was highly anisotropic. At 290 K and θ CO = 0.27 ML, the diffusion coefficient in the direction parallel to the step edges ( D ∥) was approximately 100 times greater than the diffusion coefficient perpendicular to the step edges ( D ⊥). Surface diffusion coefficients for diffusion along the terraces ( D t) and over the steps ( D s) were obtained from the measured values of d ∥ and D ⊥. For diffusion along the terrace, the surface diffusion barrier was E t=9.4±0.5kcal/mol and the diffusion preexponential was D 0,t=3.6±0.8 cm 2/s. Kinetic parameters for diffusion over a step were E s=18.1±0.8kcal/mol and D 0,s=1.4 ×10 3±0.3cm 2/s The coverage dependence of the CO surface diffusion was also measured on the stepped Ru(001) surface. As the CO coverage increased from θ CO=0.04to0.34ML, the surface diffusion coefficient at 350 K increased by a factor of 60. This coverage dependence was identical to the previously observed coverage dependence for CO diffusion on Ru(001) and was ascribed to repulsive pairwise interactions in the CO adlayer. The strong anisotropy on Ru(S)-[15(001) × 2(100)] and the identical coverage dependences on Ru(001) and Ru(S)-[15(001) × 2(100)] indicate that the steps do not trap CO, but rather act as a repulsive barrier to CO diffusion.

Site specific co-adsorption at Pt(335) as probed by infrared spectroscopy: Structural alterations in the CO adlayer under aqueous electrochemical conditions

Experiments probe the effect of hydrogen co-adsorption on the infrared spectral features of carbon monoxide adsorbed at Pt(335) {Pt(S)-[4(111)×(100)]} under aqueous electrochemical conditions. Using intermediate CO coverages, where it is possible to discern infrared spectral features for CO bound terminally (atop) at edge sites and at terrace sites, the present experiments observe greater alterations in the atop CO population at the step edge compared to the atop CO population on the terrace plane when hydrogen is co-adsorbed under aqueous electrochemical conditions. These findings suggest that hydrogen is preferentially adsorbed at step sites on the Pt(335) surface plane and they coincide with what has been revealed by recent UHV experiments which probed the effect of co-adsorbing oxygen at Pt(335) in the presence of a partial CO monolayer.

Infrared reflection–absorption spectroscopy of a model catalyst in a flow reactor: CO oxidation on Pt(100)

Infrared and mass spectral data from a continuous flow ultrahigh vacuum reactor of CO oxidation over Pt(100) are presented. Steady states corresponding to a high reaction rate branch and a low reaction rate branch are observed. For surface temperatures below 460 K, the adsorption of CO is detected in the high reaction rate branch as a new band at 1630 cm−1. A transition between the high reaction rate branch and the low reaction rate branch is seen at an unstable stationary point, where the 1630 cm−1 band disappears and vibrational bands corresponding to island growth of an adlayer of CO grow in. CO in the adlayer is adsorbed in top and bridged sites (bands at 2089 and 1880 cm−1) and the island growth is inferred from the lack of frequency shift with band growth. At surface temperatures of 460 K and greater, only bands from the adlayer of CO are observed and they are only detected in the low reaction rate branch. The transition from the low reaction rate branch to the high reaction rate branch goes through another unstable stationary point, where the disappearance of the CO adlayer is seen. A hysteresis in the CO2 production rate with CO partial pressure is observed between the high and low reaction rate branches.

Electrostatic-field effects on adsorbate bonding and structure at metal surfaces: parallels between electrochemical and vacuum systems

The observed effects of varying the surface potential, and hence the accompanying electrostatic fields, upon adsorbate bonding at ordered monocrystalline metals in electrochemical environments are summarized and discussed in comparison with field effects at related metal-vacuum interfaces for the specific case of carbon monoxide. The dependencies of the C-O stretching frequency, ν CO, upon the electrochemical surface potential, E (and interfacial field, E ), as evaluated for CO adlayers at a fixed coverage, θ; CO, and surface binding site are summarized as a function of θ CO, the metal substrate, and the double-layer ionic environment, in order to illustrate the virtues of electrochemical systems for exploring such interfacial field effects. The increasing preference for multifold versus terminal CO binding commonly observed towards lower potentials (and at larger negative fields) is also noted. These findings are compared with related field-induced effects observed for metal-CO adlayers in vacuum and also for chargeable metal-carbonyl clusters in solution. Variations in field for the former systems are induced by postdosing dipolar or ionizable coadsorbates, or by applying an external electrostatic field. Some physical differences as well as similarities in the electrostatic fields present for such systems are noted. The ν CO- E dependencies observed for the different systems are compared briefly with the predictions of theoretical models which account for potential-dependent surface-adsorbate bonding and interfacial field-induced Stark effects.

Time-resolved probes of surface dynamics

The excited vibrational state dynamics of CO adsorbed on Pt(111) are explored using subpicosecond infrared pump-probe techniques. The response for a CO( v = 1) adlayer for coverages of 0.1, 0.3, and 0.5 monolayers is measured as a function of time delay and probe wavelength. The observed transient CO vibrational spectra reveal a shift to lower frequency of the CO internal stretch mode as the degree of excitation of the adlayer is increased. These spectra are discussed in terms of density matrix models which address the coherence time of the adlayer ( v = 1) excitation and the excited state spectral characteristics of strongly-coupled anharmonic oscillators. The two-to-three picosecond recovery time of the transient response is consistent with relaxation through electron-hole pair creation. The relationship of the measured T 1 decay to the observed lineshape is addressed.

Domain wall superlattices in high coverage CO adlayers on Pd{100}

Abstract The high coverage CO overlayers on Pd{100} are found to give rise to the unusual phenomenon of continuous splitting of adsorbate-induced features in the LEED pattern over a wide coverage range. The effect is attributed to phase/antiphase domains with a smoothly varying average separation between the domain walls. Kinematic simulations indicate that the domain width distributions still remain relatively narrow and that lateral relaxation of the CO molecules at the domain walls takes place.

Domain wall superlattices in high coverage CO adlayers on Pd{100}

The high coverage CO overlayers on Pd{100} are found to give rise to the unusual phenomenon of continuous splitting of adsorbate-induced features in the LEED pattern over a wide coverage range. The effect is attributed to phase/antiphase domains with a smoothly varying average separation between the domain walls. Kinematic simulations indicate that the domain width distributions still remain relatively narrow and that lateral relaxation of the CO molecules at the domain walls takes place.

Coadsorption of C 2H 2 and CO on Ru(001): formation of mixed adlayer and the effect of CO on acetylene adsorption and decomposition

The coadsorption of CO and acetylene (C 2H 2) on Ru(001) has been investigated by LEED, TPD and HREELS. CO and C 2H 2 form an ordered mixed adlayer. A structure model for the mixed adlayer of CO and C 2H 2 is proposed. C 2H 2 in a pure layer exhibits different features at low and high coverages. C 2H 2 coadsorbed with CO shows the loss features characteristic of the low-coverage phase in a pure C 2H 2 layer irrespective of θ C 2H 2 and θ CO. The effect of coadsorbed CO on the dissociation of C 2H 2 has also been examined. The coadsorbed CO increased the decomposition temperature for ethylidyne derived from C 2H 2, while it promoted the dissociation of C x H.

Lateral interactions in CO adlayers on prismatic ZnO faces: a FTIR and HRTEM study

Lateral interactions in CO adlayers on prismatic ZnO faces: a FTIR and HRTEM study